These days, electronic cameras are increasingly widely used, and demand for solid state imaging elements (image sensors), which are central parts of them, is growing more and more. Technical development to achieve higher image quality and higher functionality is continued in the performance aspect of the solid state imaging element. On the other hand, with the spread to mobile phones, PDAs, notebook personal computers, etc., not to mention video cameras and mobile cameras, it is becoming essential for the solid state imaging element and its parts to be reduced in size, weight, and thickness in order to facilitate portability and to be reduced in cost in order to expand the spread.
In general, in a solid state imaging device, a photoelectric conversion element, an amplifier circuit, a peripheral circuit for image processing, and a multiple-layer interconnection layer for connecting elements and circuits are formed on the side of a first major surface (light receiving surface) of a silicon substrate. The solid state imaging device has a structure in which a cover glass is placed above the first major surface of a chip on which a light condensing structure of a microlens, a color filter, etc. is formed and a terminal is formed on the outer peripheral side of the first major surface or on the side of a second major surface of the chip.
To achieve higher functionality and higher speed of the solid state imaging device, the scale of the peripheral circuit is increased and also the processing speed of the peripheral circuit is increased. When it is attempted to improve the gradation expression (resolution) as a measure of increasing the image quality, it is necessary to increase the voltage. On the other hand, to achieve lower cost, it is desired to place the pixel unit and the peripheral circuit near to each other to make the chip size as small as possible.
However, in this case, since the photoelectric conversion element and the peripheral circuit are formed close to each other, issues peculiar to the image sensor occur. Since the photoelectric conversion element handles minute carriers (electrons) as a signal, it is likely that the effects of heat and electromagnetic fields from the surrounding circuit will be mixed in as noise. In addition, also minute hot carrier luminescence emitted from a transistor and a diode has a great effect on the image sensor characteristics.
The hot carrier luminescence is a luminescence that occurs due to the generation and recombination of electrons and holes generated when carriers accelerated between source and drain undergo impact ionization at the drain end, or due to the state transition of either of electrons and holes. The luminescence steadily occurs even in a transistor without any problems in characteristics, although at a low level. The amount of luminescence increases exponentially as the voltage applied to the transistor becomes higher.
The amount of luminescence is increased also when the transistor is put in high-speed operation. Since the luminescence diffuses in all directions, the effect becomes much smaller as the distance from the transistor becomes larger; but when the photoelectric conversion element and the circuit are placed very near to each other, the luminescence does not diffuse so much and a considerable number of photons are injected into the photoelectric conversion element. Since the diffusion is not sufficient, a distribution of the occurrence of hot carrier luminescence that occurs due to the differences in the density of transistors placed and the proportion of active transistors in the circuit will appear undesirably in the image as two-dimensional information. Hence, a structure designed for light blocking which is in order to suppress the amount of injection into the photoelectric conversion element to the detection limit or less is needed.
A similar effect may be given also to, not limited to the photoelectric conversion element, high-sensitivity analog elements. For example, in devices such as flash memories, since movements toward higher density and multiple-valued operations have been advanced, there is a concern that the value retained will change when noise mixing from the outside occurs.
To address such problems, in Patent Literatures 1 and 2, a light blocking structure designed to suppress the propagation of light is provided between a photoelectric conversion element and a peripheral circuit.
For example, in the technology disclosed in Patent Literature 1, as shown in FIG. 7 of Patent Literature 1, a light blocking structure having a height approximately equal to or more than the height of a photoelectric conversion element or a structure that refracts light is formed in a semiconductor substrate, and thereby the propagation of light caused by hot carrier luminescence generated from a peripheral circuit is suppressed. Furthermore, as shown in FIG. 16 of Patent Literature 1, a structure in which an anti-reflection film designed to prevent the reflection of near-infrared light is formed in order to prevent light generated in a transistor from arriving at and being reflected at the back surface side is provided.
Similarly, in the technology disclosed in Patent Literature 2, as shown in FIG. 7 of Patent Literature 2, a light blocking member is formed on the travel path of light generated in a peripheral circuit, and thereby the incidence of light on a photoelectric conversion element is suppressed.
In the technology disclosed in Patent Literature 1, as shown in FIG. 7 and FIG. 18 of Patent Literature 1, the light blocking structure is formed with a depth approximately equal to or more than the depth of the photoelectric conversion element or with such a depth as to suppress the propagation of holes generated in the peripheral circuit unit. In the case of such a structure, although components propagating in a straight line from the peripheral circuit toward the photoelectric conversion element can be blocked, light has wave components and therefore propagates by going round below the light blocking structure. That is, the light blocking structure having a depth approximately equal to the depth of the photoelectric conversion element does not provide sufficient light blocking effect, and light propagates by passing through the space below the light blocking structure. Even if the propagation of holes is successfully blocked, there is little suppression effect by the hole propagation blocking because most of the holes have recombined and changed into light components in the close vicinity of the peripheral circuit.
Patent Literature 1 provides also the anti-reflection film of near-infrared light for suppressing reflection when light caused by hot carrier luminescence has propagated up to the back surface side of the substrate. On the other hand, light generated in the transistor is radiated in all directions, and is therefore incident on the anti-reflection film with various angles. When the angle is a certain angle or less, light is totally reflected at the interface. Therefore, even when the anti-reflection film is present, it is very difficult to completely suppress the propagation of light.
In the case of a structure in which the semiconductor substrate is made thin up to approximately several micrometers, the distance from the transistor to the back surface side is significantly shortened, and light is not attenuated much and propagates up to the back surface side. Consequently, not only near-infrared light but also blue light arrives at the back surface side, and the anti-reflection film of near-infrared light cannot suppress light. In particular, in the case of a back-side illumination solid state imaging device, which has recently been developed, since the substrate needs to be made thin in order to take in light from the back surface side of the substrate, the amount of components propagating by being reflected at the back surface side is significantly increased, and the amount of light noise components is significantly increased.
Also in Patent Literature 2, a light blocking structure is provided between the peripheral circuit and the photoelectric conversion element, like in Patent Literature 1; but it is only mentioned that the light blocking structure is provided on the path of light emitted from the transistor; hence, it is very difficult to suppress components of light propagating by going round. That is, in the technology disclosed in Patent Literature 2, light propagates by passing through the space below the light blocking structure, like in Patent Literature 1.
Although the technology disclosed in Patent Literature 2 has a feature in that there is a light blocking film above the photoelectric conversion unit, this is a structure peculiar to front-side illumination solid state imaging devices and hence is not applied to back-side illumination solid state imaging devices. As described above, the amount of light noise components is significantly increased in the back-side illumination solid state imaging device; therefore, it is very difficult for the technology disclosed in Patent Literature 2 to completely suppress the propagation of light.